Recently, many new techniques for high-frequency conversion have been proposed to reduce the voltage and current stress to the component, and the switching losses in the traditional pulsewidth-modulated (PWM) converter. Among them, the phase-shifted full-bridge (FB) zero-voltage-switched PWM techniques (Dhaval B. Dalal, "A 500 KHz Multi-Output Converter with Zero Voltage Switching", APEC, 1990) are deemed most desirable for many applications because this topology permits all switching devices to operate under zero-voltage-switching (ZVS) by using circuit parasitic characteristics such as transformer leakage inductance and power device junction capacitance.
The conventional high frequency phase-shifted full bridge DC/DC converter has a disadvantage that the circulating current flows through transformer and switching devices during the freewheeling interval. The RMS current stress and conduction losses of transformer and switching devices are increased by this circulating current.
This invention solves these problems by attaching an energy recovery snubber (ERS or ERS1) to the secondary side of the transformer in the DC/DC converter. The energy recovery snubber (ERS or ERS1) of this invention has three fast recovery diodes Ds.sub.1, Ds.sub.2 and DS.sub.3, two resonant capacitors Cs.sub.1 and Cs.sub.2, and a small resonant inductor Lr which can be ignored because the transformer leakage inductance L.sub.l is used instead of inserting the resonant inductor Lr.
FIG. 1 shows a prior art full bridge DC/DC converter schematic circuit and relevant wave-forms thereof are shown in FIG. 2 and FIG. 3. The circuit includes parasitic elements such as body diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4, junction capacitance Cp across each switching device, leakage inductance L.sub.l, and magnetizing inductance Lm of the transformer. In the case of regular PWM control, shown in FIG. 2, until the time at t.sub.0, the energy is delivered from the source to the load through switches Q.sub.1 and Q.sub.2. When the switches Q.sub.1 and Q.sub.2 are turned off, the load current I.sub.0 flows through rectifiers D.sub.5 and D.sub.6 during the freewheeling interval t.sub.0 -t.sub.1 or t.sub.2 -t.sub.3. Then, the transformer primary current (I.sub.1 (t)) becomes zero.
The main problem with this operating sequence is that when all four switches are turned off (t.sub.o, t.sub.2), the energy stored in the leakage inductance of the power transformer causes severe ringing with the junction capacitances of the switching devices.
To minimize the parasitic ringing as shown in FIG. 3, the gate signals for switches Q.sub.2 and Q.sub.4 are delayed (phase-shifted) with respect to those of Q.sub.1 and Q.sub.3, so that during the time interval t.sub.2 -t.sub.3 and t.sub.7 -t.sub.8 when the secondary voltage is zero, one of the primary switches is always left on. This provides a low-impedance path for the current of the transformer leakage inductance L.sub.l to circulate, thus solving the problem of the parasitic ringing associated with the conventional PWM control hard-switching FB converter (FIGS. 1 and 2).
However, when switch Q.sub.1 is turned off at time t.sub.1 (switch Q.sub.3 at time t.sub.6), the primary current I.sub.1, which is the sum of the reflected output current nI.sub.o and the transformer primary magnetizing current Im, circulates through Q.sub.2 and D.sub.3 during freewheeling mode t.sub.1 -t.sub.3 and decrease with a slope of the following equation (1): ##EQU1## wherein n is a turns ratio of the transformer given as n=Ns/Np.
Due to this circulating current, RMS current stress, conduction losses of the transformer and switching devices are increased. The overall efficiency is also reduced.